1. Field of the Invention
The present invention relates generally to color cathode ray tube (CRT) display apparatus and more particularly to gamma factor correction to automatically and independently adjust the cathode drive voltage of the cathode for each of the color phosphors in accordance with each of the phosphor's light emissive characteristics at a variable reference brightness.
2. Description of the Prior Art
In most prior art color CRT display systems employing a multi-gun CRT, such as, for example, home and commerical television receivers, where normal viewing ambient light conditions do not vary significantly, essentially fixed predetermined drive voltages for the red, green, and blue cathodes are used. Thus, any change in the manual brightness setting will cause only a DC shift while maintaining the same relative voltages applied to the CRT. However, since the color gun response is a nonlinear function of the applied DC cathode voltage, the relative color balance of a multi-gun structure is disturbed. Thus, to restore the proper color balance, compensating readjustments of the drive to the red, green, and blue guns individually are necessary. In the applications just described, since the adjustments are normally made over relatively narrow ranges of ambient light conditions, the color shift is slight and generally not corrected. Similarly, the automatic brightness function found on commercial television receivers affects the drive of all three guns in identically the same manner and has no feature to compensate the relative shift in color balance, but again the restricted operating range keeps the error from being objectionable. However, these conventional color CRT brightness controls, whether automatic, manual, or both, are unsuitable for critical applications of color CRTs such as where used to display information in an aircraft cockpit environment, wherein the ambient light level may vary uncontrollably over wide ranges, requiring readjustment of the display brightness over a wide range.
Conventionally, a multi-gun CRT, such as the shadow-mask type, includes red, green, and blue cathodes for emitting the three electron beams which excite the corresponding red, green, and blue phosphor triads through the screen apertures, the output light intensity of each phosphor, in Foot-Lamberts, varying in accordance with the spectral response of any filter on the screen, and non-linearly in accordance with the voltage applied to each cathode in a determinable manner, such ratio being a function of a brightness factor known as gamma (.gamma.) for each primary color, and which may be a variable from tube to tube as well as color to color.
Therefore, in order to maintain a given color hue or chromaticity over the entire color spectrum and brightness range, the relative intensity of each primary color component must be varied in accordance with its particular gamma characteristic. In addition it has been found necessary to modify each color hue component in accordance with the square of the video driving voltage to provide color tracking.
One example of an automatic color correction and brightness tracking device is in U.S. application Ser. No. 304,451, filed June 22, 1981 and assigned to the Assignee of the present invention. The invention therein discloses an apparatus for automatically sensing and correcting for ambient brightness levels and display writing modes, for calculating the corresponding brightness level required for each of the primary color components of a commanded color, and for determining the required drive voltages to the cathode ray tube cathodes. These operations require a relatively sophisticated microprocessor control system and an associated personality programmable-read-only-memory (PROM) containing the color-brightness characteristics of the particular cathode ray tube to which it is dedicated. While capable of a wide range of brightness in color tracking correction, relatively complex circuitry is required for mathematical computations control, and storage. The complexity of the prior art disclosure was necessitated in part by the need to operate and perform the gamma and contrast corrections at a video pulse rate in the megaHertz range.
The present invention performs the function of correcting for the color gun non-linearity while keeping cost and complexity to a minimum by avoiding the need to perform non-linear corrections and mathematical computations at video speeds, thus permitting relatively simple operational amplifiers for an analog system, and permitting relatively simple analog-digital and digital-analog converters and storage for a digital correction control system. A hybrid system comprised of analog color selection and digital color correction is also described.